1. Field of the Invention
The present invention relates to a vapor deposition coating material having a high heat resistance and a high thermal shock resistance and being capable of forming a stable melt pool in vapor deposition for forming a coating film.
2. Description of the Related Art
Heat-resistant coating films are generally formed by flame spray coating. With the demand for improved properties of the coating films and as the result of the progress of the vapor deposition technique, physical vapor deposition (hereinafter referred to as PVD) has come to be practically employed for formation of heat-resistant coating films. Especially, PVD processes employing an electron beam (EB-PVD) are attracting attention, and various methods have been developed for forming a heat resistant coating film by the EB-PVD process as disclosed in ADVANCED MATERIALS & PROCESS, Vol. 140, No.6, p.18-22 (1991).
The aforementioned EB-PVD process for heat-resistant film formation is employed for heat-resistant coating of parts of aircraft engines and like parts. The vapor deposition material for coating of engine parts is required to have sufficiently high heat resistance to withstand the direct contact with a high-temperature combustion gas, for which the material should have sufficiently high melting points and a sufficiently high purity. The coating material for engine parts is required also to be sufficiently adhesive to metal parts, not to exfoliate in heating cycles, not to be corroded by a combustion gas component, to have low thermal conductivity, and to maximize the lowering of the surface temperature of the metal parts for improving the durability of the metal parts to achieve the object of the coating film. Few kinds of materials are available to meet the above requirements. Molded bodies of zirconia powders containing a stabilizer such as yttria are used for such purpose.
Such a vapor deposition material composed of zirconia containing a stabilizer, which has a high melting point, is formed into a film by EB vapor deposition employing a high-energy electron beam for the purpose of forming the vapor deposition film at a high-speed. In the EB vapor deposition method, a vapor deposition material placed in a crucible is irradiated with a high-energy electron beam violently, whereby a usual molded body of the vapor deposition material is broken by thermal shock. The breakdown of the vapor deposition material body hinders the stable feed of the vapor of the deposition material to lower the quality of the vapor-deposited film. Therefore, the vapor deposition material body should not be broken by the electron beam in the practice of vapor deposition.
Various vapor deposition material bodies are disclosed which are not broken by the shock of the electron beam. To improve the heat resistance and the thermal shock resistance against the electron beam, a method is disclosed in which the vapor deposition material body is made porous.
For example, DE4302167C1 discloses a vapor deposition material body composed of zirconia containing as a stabilizer yttrium at a content ranging from 0.5 to 25% by weight, being made porous to have a density ranging from 3.0 to 4.5 g/cm.sup.3.sub.1, and containing monoclinic phase at a phase ratio ranging from 5 to 80%, whereby the breakdown caused by EB irradiation is prevented. This vapor deposition material, however, does not have sufficient heat resistance and thermal shock resistance, and the material body is cracked by violent heating by high-energy EB irradiation to render steady operation impossible.
JP-A-7-82019 discloses a vapor deposition material for heat resistant coating film, formed from a zirconia-containing porous sintered body. This sintered body is prepared by granulating a mixture of particulate zirconia having a purity of 99.8% or higher and a particle diameter ranging from 0.01 to 10 .mu.m, and particulate yttria having a particle diameter of 1 .mu.m or less to have granules of a diameter ranging from 45 to 300 .mu.m constituting 70% or more of the entire granules, heat-treating the granules to obtain a zirconia-containing sintered body containing granules of diameters from 45 to 300 .mu.m diameter at a content of 50% or more. The sintered body has a porosity ranging from 25 to 50%, and 70% or more of the pores are in a pore diameter range from 0.1 to 50 .mu.m. A thermal shock resistant and heat resistant coating film can be prepared from this vapor deposition material. However, this vapor deposition material, in which solid solubilization of the stabilizer tends to proceed, shrinks remarkably on EB irradiation. Therefore, the material comes to be cracked by shrinkage around the portion melted by EB irradiation, which renders the vapor deposition process unstable.